A medical device can be implanted in a body to perform one or more tasks including monitoring, detecting, or sensing physiological information in or otherwise associated with the body, diagnosing a physiological condition or disease, treating or providing a therapy for a physiological condition or disease, or restoring or otherwise altering the function of an organ or a tissue. Examples of an implantable medical device can include a cardiac rhythm management device, such as a pacemaker, a cardiac resynchronization therapy device, a neural stimulation device, a neuromuscular stimulator, or a drug delivery system, among others.
Neural stimulation or monitoring devices can be configured to deliver therapeutic pulse signals to nerve tissue to evoke a patient response, such as a cardiac response. The sympathetic and parasympathetic nervous systems can be modulated using neural stimulation, such as to protect against cardiac remodeling or predisposition to fatal arrhythmias after a myocardial infarction. In an example, neural stimulation can include autonomic modulation therapies (AMT) comprising stimulating neural targets in the autonomic nervous system, such as to treat disorders where autonomic imbalance can occur, such as in heart failure.
In an example, a medical device can be configured to monitor one or more patient physiological parameters, such as thoracic impedance, pulse transit time, relative pulse pressure, or changes in blood vessel geometry. In one example, Friedman et al., in U.S. Pat. No. 6,648,828, entitled “CONTINUOUS NON-INVASIVE TECHNIQUE FOR MEASURING BLOOD PRESSURE USING IMPEDANCE PLETHYSMOGRAPHY,” refers to using impedance plethysmography to measure the impedance at two locations on a limb of an animal to detect when a blood pressure pulse occurs at those two locations. Impedance plethysmography can include applying an electrical current to body tissue and monitoring changes in voltage, an example, voltage changes can correspond with changes in body fluid volume.
In another example, Hayes et al., in Vol. 31, No. 6, November/December 2007, of the Journal of Medical Engineering and Technology, entitled “THE RELATIONSHIP BETWEEN VASCULAR EXPANSION OF THE AORTA AND PULMONARY ARTERY AND THE GENESIS OF THE IMPEDANCE CARDIOGRAM USING THE TECHNIQUE OF SONOMICROMETRY,” refers to using impedance electrodes placed around a canine aorta to determine a contribution of vascular expansion of the aorta on an impedance cardiogram. Sonomicrometry can include applying ugh frequency vibrational energy ultrasound) to body tissue. For example, piezoelectric transducers, such as disposed in or on body tissue, can be used as ultrasound transmitters and receivers. The distance between the transducers can be determined by analyzing a propagation time of the ultrasound signal.
Pulse transit time and relative pulse pressure can be determined using various sensors disposed in or on a patient body to indicate, among other things, cardiac output or blood vessel compliance. For example, Kounalakis et al., in Vol. 9, August, 2009, of Cardiovascular Engineering, entitled “THE ROLE OF PULSE TRANSIT TIME AS AN INDEX OF ARTERIAL STIFFNESS DURING EXERCISE,” notes that pulse transit time, as an index of functional vessel stiffness, can be affected by changes in cardiac output.
Implanted electrodes can be used to deliver electrical stimulation signals to areas near blood vessels, nerves, or other internal body locations. In an example, an electrode implanted in a cervical region (e.g., at or near a neck region) can be used to measure dimensional changes in an artery using impedance plethysmography. Measured artery dimensional changes can be used to determine one or more physiological parameters associated with a patient health status, such as pulse transit time, relative pulse pressure, or arterial compliance, among others. These parameters can be used to monitor a patient health status or to modulate a patient's therapy, among other uses. In an example, an electrode configured to deliver an electrostimulation signal to nerve tissue can be used to provide non-neurostimulating electrical stimulation plethysmography signals near a blood vessel.
The present inventors have recognized, among other things, that a problem to be solved can include monitoring a patient health status using impedance measurements to monitor relative pulse pressure or pulse transit time, among other physiological indicators. In an example, the present subject matter can provide a solution to this problem, such as by using plethysmography analysis techniques to process impedance information received from cervical body locations. In an example, the present subject matter can include using impedance plethysmography information to discern cervical blood vessel dimensional changes.
The present inventors have recognized, among other things, that another problem to be solved can include reducing a stimulation energy delivery frequency, such as to prolong battery life in an implantable device, or to reduce the number of energy deliveries provided to a patient body. In an example, the present subject matter can provide a solution to this problem, such as by incorporating impedance plethysmography pulse signals with neural stimulation pulse signals.
This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application.